T cells play a central role in immune response. The first step in the signaling mechanism of the T cell receptor (TCR) is the activation of Src family of tyrosine kinases--Lck and Fyn. Although much is known about signal transduction mechanism of TCR, the exact molecular mechanism of Lck and Fyn activation is unknown. Through yeast two-hybrid screening we have recently cloned a novel SH3 ligand called Unc119. In preliminary results we show that Unc119 is associated with the TCR complex (CD3 and CD4) activates Lck and Fyn in vitro and in vivo. Unc119 deficient cells are unable to activate Lck and Fyn, fail to produce IL-2 and proliferate poorly. The objective of this research proposal is to study the signaling function of Unc119 for T cell function and the role of Unc119 in the pathogenesis of idiopathic CD4 lymphopenia, a rare immunodeficiency disorder. Our specific aims are 1). To study the molecular mechanism of Unc119 activation of Lck/Fyn. 2). To examine the importance of Unc119 for T cell differfentiation and function. 3). To investigate the role of Unc119 in the pathogenesis of idiopathic CD4 lymphopenia. We will map the CD4 and kinase (Lck/Fyn) binding sites of Unc119 through mutational approaches and study the importance of these sites for kinase activation. In order to establish the biological relevance, we will generate Unc119 knockout mice and study thymopoiesis and T cell function. We have identified one ICL patient with Unc119deficiency and impaired Lck activation. This patient has an Arg50-->Lys mutation in the coding sequence and has another mutation in the 3' untranslated region. We will screen ICL patients nationwide through preestablished collaboration and examine the presence of this and other mutations. We will examine the functional relevance of these mutations by studying translation and decay of the protein. The biological relevance will be studied by expressing the mutated gene in normal T cells and vice versa. The proposal is important because it describes the identification and characterization of a novel activator of TCR-associated tyrosine kinases. Further, it examines the molecular mechanism of idiopathic CD4 lymphopenia, which may pave the way to gene therapy for this rare immunodeficiency disorder.